In recent years, an increase in size of a wafer is demanded, and a wire saw is mainly used to slice an ingot with this increase in size.
The wire saw is a apparatus that allows a wire (a high-tensile steel wire) to travel at a high speed and presses an ingot (a work) against the wire to be sliced while applying a slurry to the wire, thereby slicing the ingot into many wafers at the same time (see Japanese Unexamined Patent Publication (Kokai) No. 262826-1997).
Here, FIG. 6 shows an outline of an example of a general wire saw.
As shown in FIG. 6, a wire saw 101 mainly includes a wire 102 that slices an ingot, grooved rollers 103 (wire guides) around which the wire 102 is wound, a mechanism 104 that gives the wire 102 a tensile force, a mechanism 105 that feeds the ingot to be sliced, and a mechanism 106 that supplies a slurry at the time of slicing.
The wire 102 is unreeled from one wire reel 107 and reaches the grooved rollers 103 through the tensile-force-giving mechanism 104 formed of a powder clutch (a constant torque motor 109), a dancer roller (a dead weight) (not shown) and so on through a traverser 108. The wire 102 is wound around this grooved rollers 103 for approximately 300 to 400 turns, and then taken up by a wire reel 107′ through the other tensile-force-giving mechanism 104′.
Further, the grooved roller 103 is a roller that has a polyurethane resin press-fitted around a steel cylinder and has grooves formed at a fixed pitch on a surface thereof, and the wire 102 wound therearound can be driven in a reciprocating direction in a predetermined cycle by a driving motor 110.
It is to be noted that such an ingot-feeding mechanism 105 as shown in FIG. 7 feeds the ingot to the wire 102 wound around the grooved rollers 103 at the time of slicing the ingot. This ingot-feeding mechanism 105 includes an ingot-feeding table 111 that is used to feed the ingot, an LM guide 112, an ingot clump 113 for grasping the ingot, a slice pad plate 114, and others, and driving the ingot-feeding table 111 along the LM guide 112 under control of a computer enables feeding the ingot fixed at the end at a previously programmed feed speed.
Moreover, nozzles 115 are provided near the grooved rollers 103 and the wound wire 102, and a slurry can be supplied to the grooved rollers 103 and the wire 102 from a slurry tank 116 at the time of slicing. Additionally, a slurry chiller 117 is connected with the slurry tank 116 so that a temperature of the slurry to be supplied can be adjusted.
Such a wire saw 101 is used to apply an appropriate tensile force to the wire 102 from the wire-tensile-force-giving mechanism 104, and the ingot is sliced while causing the wire 102 to travel in the reciprocating direction by the driving motor 110.
On the other hand, in a wafer, a size of a surface waviness component that is called “nano-topography” is a problem in recent years. This nano-topography is obtained by taking a wavelength component having a wavelength λ=0.2 mm to 20 mm that is shorter than “Sori” or “Warp” and longer than “surface roughness” out of a surface shape of a wafer. And, this nano-topography is very shallow waviness having a PV value of 0.1 μm to 0.2 μm or below. It is said that this nano-topography affects a yield of an STI (Shallow Trench Isolation) process in device manufacture.
Although the nano-topography is produced in a wafer processing step (slicing to polishing), it was revealed that a nano-topography caused due to wire saw slicing (i.e., slice waviness) can be classified into three types, i.e., “one that is extemporaneously produced”, “one that is produced in a position where slicing is started or ended”, and “one having a periodicity” as shown in FIG. 8.
Of these types, one that is produced in “slicing start/end portion of a wafer” has a high rate that it is rejected in a numeric judgment regarding a nano-topography. In particular, a nano-topography in the “slicing end portion” has a higher rate than a nano-topography in the “slicing start portion”. And the “slicing end portion” highly frequently becomes a position making a numeric value regarding a nano-topography the worst in a wafer radial direction or the nano-topography in the “slicing end portion” is rejected in the numeric judgment, and hence improvement is strongly demanded.